The present invention relates to a nonvolatile semiconductor disk device (hereinafter referred to as a xe2x80x9cdisk cardxe2x80x9d) as one of peripheral function extender cards of a personal computer (hereinafter abbreviated to a xe2x80x9cPCxe2x80x9d), etc., and also relates to writing control to this disk card.
The disk card as a peripheral device of the PC is stored with data.
Then, the disk card is capable of holding a content of the storage without requiring a power supply.
Next, a nonvolatile semiconductor memory such as, e.g., a flash memory is employed as a storage medium of the disk card.
This nonvolatile semiconductor memory is stored with the data in such a form as to be formatted to a fixed size called a sector as in the case of the disk device like a flexible disk and a hard disk.
Incidentally, the disk card in a name card size becomes, with increases by leaps in storage capacity of the semiconductor memory, capable of storing the data of several tens of Mega bytes.
This disk card is used for storing data about a picture photographed by, e.g., a digital camera in the way of utilizing merits of being small in size but large in capacity and of the storage content being held even when switching off the power supply.
Next, the disk card with a completion of the photography is taken out of the digital camera and set in the PC, and the image data stored thereon can be read and digitally processed.
FIG. 1 is a diagram showing one example of a conventional disk card.
This disk card includes an interface unit 10 connected to a host 1 such as the digital camera and the PC, a central processing unit (hereinafter abbreviated to a xe2x80x9cCPUxe2x80x9d) 20 for executing whole control within the disk card by transmitting and receiving a variety of control signals to and from this host 1, a disk control unit 30 for controlling a transfer of the data to the host 1, an internal bus 40 through which to transfer the data inwardly the disk card, and a storage unit 50 for storing the data.
Then, the disk control unit 30 has a buffer memory 31 for temporarily holding sector-basis data given from the host 1.
The sector contains e.g., 536-bytes data in such a fixed format that a header portion containing data about a validity, etc. of this sector and a correction code for correcting an error are added to, e.g., 512-bytes data.
The disk control unit 30 incorporates a function to write sector-basis data to the corresponding storage unit 50 via the internal bus 40 on the basis of an address signal given via the interface unit 10, and to read the sector-basis data stored in the storage unit 50.
The storage unit 50 is constructed of a plurality (e.g., 15 pieces) of memory chips 50a, 50b, . . . , 50n connected in common to the internal bus 40.
Addresses different from each other are allocated to these memory chips 50a-50n. 
Then, each of the memory chips 50a-50n has the same construction, and includes a buffer memory 51 for temporarily holding the sector-basis data and a nonvolatile semiconductor memory 52 for storing the sector-basis data.
The nonvolatile semiconductor memory 52 is capable of holding a content of the storage even if a supply of the power supply is stopped.
Each of the memory chips 50a-50n has a memory control unit 53 for controlling a transfer of the sector-basis data between the buffer memory 51 and the nonvolatile semiconductor memory 52.
Next, in this disk card, when the host 1 issues a command to write the data, the write data is temporarily held in the buffer memory 31 within the disk control unit 30 via the interface unit 10.
The data held in the buffer memory 31 is transferred to and held in the buffer memory 51 in one of the memory chips 50i (however, i=a to n) which corresponds to the address thereof via the internal bus 40.
The data held in the buffer memory 51 in the memory chip 50i is written to a predetermined storage area in the nonvolatile semiconductor memory 52 under the control of the memory control unit 53.
At this time, a transfer time of the data transferred from the host 1 to the buffer memory via the interface unit 10 and the buffer memory 31, is on the order of several hundred xcexcs.
On the other hand, for instance, a time of several ms is required for writing the data temporarily held in the buffer memory 51 to the semiconductor memory 52.
For this purpose, the storage unit 50 is divided into a plurality of memory chips 50a-50n, and each memory chip, e.g., 50a is provided with the buffer memory 51 and the nonvolatile semiconductor memory 52.
Then, the data is independently written to the nonvolatile semiconductor memory 52 from each of the buffer memories 51. With this operation, there can be substantially equivalently executed the writing process to the disk card from the host 1.
On the other hand, under the control of the CPU 20, when the host 1 issues a command to read the data, a reading command is given to the memory chip 50a stored with the data to be read.
Then, the sector-basis data is read from the corresponding storage area in the nonvolatile semiconductor memory 52.
The thus read data is temporarily held in the buffer memory 51 and thereafter held in the buffer memory 31 within the disk control unit 30 via the internal bus 40.
The data written to the buffer memory 31 is further transferred to the host 1 via the interface unit 10.
There arise, however, the following problems inherent in the prior art disk card.
In the disk card, the storage unit 50 is divided into the plurality of memory chips 50a-50n in order to substantially equivalently hold an access speed for high-velocity writing and reading processes in the interface unit 10.
Next, each of the memory chips 50a-50n is provided with the buffer memory 51.
Then, the disk card is capable of equivalently executing the writing operations at the high speed by executing the writing operations to the memory chips 50a-50n in parallel.
An electric current necessary for the writing operation per memory chip is on the order of, e.g., 15 mA.
A total operation current, when the number of the memory chips 50a-50n in the process of the simultaneous writing operations increases, becomes large.
Accordingly, only the storage unit 50 requires a current of approximately 150 mA when ten pieces of memory chips 50i are in the simultaneous writing operations.
Therefore, the host 1 must include a power supply having a current capacity allowing for it.
The thus constructed disk card is used not only simply as a peripheral device of the PC but also for storing data about photographed picture in such a way as to be attached to, e.g., a digital camera.
The digital camera is driven by a battery and therefore has a limit in terms of being supplied with a large current when writing the image data.
It is a primary object of the present invention, which was contrived to obviate the problems inherent in the prior art described above, to provide a disk card requiring no large current of a power supply by restricting the number of memory chips 50a-50n in the process of simultaneous writing operations.
To accomplish the above object, a nonvolatile semiconductor disk device according to the present invention is a disk card comprising an interface unit for transferring data given from outside, a plurality of memory chips each including a nonvolatile semiconductor memory for storing data and a buffer memory for temporarily holding the data to write the data to the semiconductor memory, and a control unit for outputting the data transferred via the interface unit, reading the data from a corresponding memory chip in accordance with a designation given from outside and outputting the data to the interface unit. The control unit monitors the number of simultaneous writing processes that are simultaneously being executed in the plurality of memory chips, and controls the outputs of the data given from outside to the corresponding memory chips so that the number of simultaneous writing processes does not exceed a predetermined number.
Next, in the nonvolatile semiconductor disk device, the control unit has a function added thereto, to output the data to the memory chips and thereafter to start monitoring a completion of the writing processes to the memory chips after an elapse of a fixed time substantially corresponding to a necessary writing time in the memory chips.
Then, the nonvolatile semiconductor disk device is so constructed as to take a card-like configuration as a disk card and to be attachable and detachable to the processor through the interface unit.
The nonvolatile semiconductor disk device, since the disk card is constructed as described above, exhibits the following operations.
The control unit, when the data is transferred from the processor via the interface unit, checks the number of memory chips that are now in the process of the writing operations.
Then, the control unit outputs the data to the relevant memory chips if the number of the memory chips in the writing processes is less than a predetermined number.
The control unit, if the number of the memory chips in the writing processes is the predetermined number, does not output the data to the relevant memory chips till the number of the simultaneous writing processes becomes less than the predetermined number.
Then, the control unit outputs the data to the memory chips just when the number of the simultaneous writing processes becomes less than the predetermined number.
Next, the nonvolatile semiconductor disk device exhibits the following operations.
The control unit checks the number of the memory chips that are now in the writing processes when the data is transferred from the processor via the interface unit.
Then, the control unit, if the number of the memory chips in the writing processes is less than the predetermined number, outputs the data to the relevant memory chips.
Subsequently, the control units, after an elapse of a fixed time substantially corresponding to a necessary write time, starts monitoring whether or not the writing processes are completed.
On the other hand, the control unit, if the number of the memory chips in the writing processes, does not output the data to the relevant memory chips till the number of the simultaneous operations becomes less than the predetermined number.
Then, the control unit, if the number of the simultaneous writing operations is less than the predetermined number as a result of monitoring the completion of the writing processes, outputs the data to the memory chips, and the data is written to the semiconductor memory in the memory chip.